This invention pertains to a ceramic slip composition and more particularly to a slip having low viscosity and Newtonian behavior for use in a curtain coating step in a method for making monolithic ceramic capacitors.
Monolithic ceramic capacitors are made by interleaving electrode films between adjacent layers of green ceramic forming a stack. The stack is fired to burn out organic binders and to sinter the ceramic to maturity. Alternate ones of the electrodes extend to one end of the sintered body while the other electrodes usually extend to an opposite body end. Terminals are formed at each of these body ends to contact the two alternate sets of buried electrodes as is more fully described, for example, in U.S. Pat. No. 4,027,209 to G. Maher, issued May 31, 1977 and assigned to the same assignee as is the present invention.
The production of green ceramic layers by present day manufacturers of monolithic ceramic capacitors is accomplished mainly by two methods. The most prevalent is known in the industry as the "tape" process whereby a ceramic slurry is extruded under hydraulic pressure onto a moving carrier on a conveyor belt. The other is known in the industry as the curtain coating process whereby a falling sheet or curtain of a ceramic slurry is interrupted and carried away in a central portion by a moving carrier on a conveyor belt.
The tape process is illustrated and further described by Howatt in U.S. Pat. No. 2,582,993 issued Jan. 22, 1952 and by Lindquist in U.S. Pat. No. 3,660,328 issued May 2, 1972. On the other hand the flip process is further described by Hurley et al. in U.S. Pat. No. 3,717,487 issued Feb. 20, 1973 and by Coleman in U.S. Pat. No. 4,060,640 issued Nov. 29, 1977. The later two patents pertain directly to the present invention, are incorporated by reference herein, and are assigned to the same assignee as is the present invention.
Both the tape and the curtain coating process use a ceramic slurry or paint. These ceramic paints have a pseudoplastic rheology, i.e. the viscosity of the paint increases with decreasing shear rate, just as for house paints. House paints are required to work or brush easily but after application the still paint is expected to set without running. For making multilayer capacitors this has the particular disadvantage that each layer of ceramic has the same thickness over electroded regions, leading to a central bump in the stack corresponding to the underlying electrodes. This makes it difficult to make capacitors conform to certain requirements for uniform dimensions. Deposition of electroding ink to each dielectric layer becomes progressively more difficult as the surface becomes more uneven. This can adversely affect capacitor properties such as capacitance tolerance and breakdown strength.
The occurrence of pin holes in monolithic ceramic capacitor dielectrics is an additional problem for capacitors made using the prior art processes. Under some circumstances, pin holes in capacitors made by curtain coating occur such that specifications relating to dielectric strength or leakage current cannot be met with high yields. This may be alleviated by using a mixed solvent system in the slip as described by Levinson in U.S. Pat. No. 3,125,618 issued Mar. 17, 1964 or by using a smaller quantity of organic binder in the slip. Such curative steps, however, tend to give a weaker green ceramic body that is prone to crack during the cutting apart of the individual green capacitor bodies. These pin-holing and green-strength problems are even more acute when finer powders are used. In pseudoplastic materials, the fines dictate the properties of the suspension.
It is an object of the present invention to provide monolithic ceramic capacitors having higher quality, namely uniformity of dimensions and electrical properties and with better physical integrity, e.g. fewer pin holes and cracks.
It is a further object of this invention to provide an improved method for making such higher quality capacitors including a curtain-coating step employing a low viscosity ceramic slip having Newtonian rheology, i.e. a viscosity independent of shear rate.